The present invention relates to a new technique of configuring a satellite antenna array and transponder to provide either a global beam or simultaneous global and spot beams as desired, thus increasing the capacity of traffic the satellite can handle while minimizing the required amount of additional hardware. The technique will be described below in detail with reference to a particular application (the INMARSAT II satellite system), but is generally applicable to any large thinned arrays.
Telephony for aeronautical and maritime applications where the mobile earth station antenna gain is very low requires a very igh equivalent isotropic radiated power (e.i.r.p.) This requirement can severely limit the capacity which a satellite can handle, and thus can limit the amount of service which the satellite can provide. Accordingly, various approaches to modification of satellite design to provide an additional spot beam channel have been considered. The present invention has been achieved as a result of these considerations.
The INMARSAT II system provides a useful context for describing the development and implementation of the invention. The global L-band transmit antenna for the INMARSAT II system uses a 61-element array of cup dipoles, 43 of which are excited to provide a shaped global beam. 18 of the elements normally are not excited, as will be discussed below, to provide the global beam.
A configuration of the transponder employed is shown in FIG. 1. The C/L transponder 10 shown uses a C band global array antenna 20 to receive the uplinks from coastal earth stations. A low noise receiver 100, comprising 6/1.5 GHz receivers 110, amplifies and downconverts the received C band signal to L band. An automatic level control (ALC) unit 200, including individual ALC circuits 210, limits the output to the high power amplifier (HPA) section 300. The L band HPA section 300 accepts the 1.5 GHz signal from the ALC 200 and divides it equally, via a passive hybrid network which is part of a signal divider 305, among inputs of four parallel-connected linearizers 310/traveling wave tube amplifiers (TWTAs) 320 arranged in two three-for-two redundant schemes. Outputs of four active HPAs are combined via another passive hybrid network which is part of an L-band power combiner 410, and the resulting signal is fed to an L band transmit array 500.
A sample configuration of the 61-element array 500, with excitation coefficients for the 43 elements, is shown in FIG. 2. In FIG. 2, there are five rings R.sub.1 -R.sub.5 of elements 1, with the center element constituting a ring by itself. Thus, counting out from the center, FIG. 2 shows the fourth ring as having elements which normally are not excited. These elements are terminated in matched loads to preserve the mutual coupling environment in the array. A sample global array pattern for the excitation coefficients of FIG. 2 is shown in FIG. 3 which is calculated based on an embedded element pattern measured at COMSAT Laboratories, also is shown in FIG. 3. The global edge of coverage directivity is about 19.2 dB.
As presently configured, the INMARSAT II system has three channels in the C/L transponder 10 (channels 2, 3, and 4). In attempting to provide a spot beam capability, the use of two channels (channels 2 and 4) was considered. These channels currently use approximately equal amplifier power outputs (45 watts), and channel 3 uses a total amplifier power output of about 90 watts, so that the output of channel 3 is substantially equal to the combined output of channels 2 and 4. The basic spot beam arrangement considered was a fixed or scannable beam having a width of about 7.degree. in the east-west direction. A typical example is shown in FIG. 4, which shows a North Atlantic beam 5 covering the area between the North Sea (to the east) and Newfoundland (to the west).
Of the several alternatives considered, most were thought not to satisfy either cost-performance, weight-performance or complexity-performance trade-off limitations. For example, one approach which contemplated providing a separate reflector antenna to generate the spot beam, would not have required any change in the presently-configured global antenna array. However, in order for the separate antenna to be able to produce more than one beam with desired spatial isolation, a more complicated feed array and beam forming network would have been required. The diameter of the reflector also would have had to be increased to satisfy the isolation requirements.
The addition of a separate beam forming network while using the same 61-element array, also was considered, but was discarded because of problems arising from difficulty of adequate separation of global and spot beams. While using the complete array for global and spot beams would result in savings in volume taken up by the system, the global and spot beams would have to be separated either by frequency or by polarization. While achievable with current technology, this requires the use of diplexers, polarizers, or combiners behind every element, thus adding additional weight, and resulting in net loss of signal strength.
A third approach considered was to use the 18 elements in the fourth ring by itself without exciting the other 43 elements. However, the gain of this configuration was found to be too low, and further resulted in very high sidelobe levels.